Report 2: Mendel Law

Learning Outcomes:

At the end of experiment, we are able to:

1.      Recognize and interpret maize that illustrated Mendel’s both law.

2.      To state the reasons either the result is relevant or not.

Introduction:

In many years ago, before Gregor Mendel published a paper detailing a set of experiment on the common garden pea, there are some people already start to think about the genetic inheritance. For example, Aristotle proposed the theory of pangenesis. In the seventh century, Aton van Leeuwenheck “observed” the homunculus, who believe that human is from “small men”. Also during the seventh century, Regnier de Graaf  proposed that the egg contains an entire human in miniature and that semen  only stimulates its growth. Lastly, a modern genetics began in the 1860's with the experiments of an Augustinian monk named Gregor Mendel who discovered the fundamental principles of inheritance.

There are some method that Mendel’s use during his investigation. Mendel adopted a quantitative approach to his experimentation. Why Mendel use garden peas? Garden peas have many advantages such as they are easy to grow, available in many easily distinguish varieties and also strict control over mating was possible(small bags over flowers prevented cross-pollination to insure the parentage of new seeds.

Mendel not only applied quantitative methods but also chose traits which differed in a relatively clear-cut manner. Mendel chose seven characteristics, each of which occurred in two forms: seed shape (round or wrinkled), seed color (yellow or green), seed coat color (gray or white), pod shape (inflated or constricted), pod color (green or yellow) flower position (axial or terminal), stem length (tall or dwarf).Mendel observed the transmission of selected traits for several generations and arrived at two principles of heredity which are now known as the law of segregation and the law of independent assortment.

The first law is the law of segregation. When Mendel crossed true-breeding plants exhibiting different forms of a trait, green-pod and yellow-pod color, he bound that the traits did not blend, but that the F₁ progeny (offspring) produced only green-colored pods. Mendel hypothesized that if the heritable factor for yellow pods had been lost, then the F₁ plants should only be capable of producing green-pod progeny. Mendel allowed the F₁ plants to self-pollinate to produce the next generation. In this F₂ generation, there were 428 plants with green pods and 152 plants with yellow pods which gives a 3:1 ratio of green to yellow. From these types of experiments and observations, Mendel concluded that the heritable factor for yellow pods was not lost in the F₁ plants, but was masked by the presence of the green-pod factor. We now call these heritable factors genes.

In this experiment, we use maize to look details about the Mendel’s law. We use for type of maize.

Materials:

            Ear of maize C₁,C₂,C₃ and C_4.

Method:

            We count number of kernels for each phenotypes of the ear of maize (C₁,C₂,C₃ and C_4.) and record it the four different tables below according the description given.

Result:

Monohybrid crosses in maize

Record for C₁ kernels.

Phenotype	Number of kernels observed (O)	Number of kernels expected (E)	Genotypes of kernels
Purple color	547	555	C_
Yellow color	192	185	Cc
Total	739	739

Dihybrid Crosses in maize

Record for C₂ kernels.

Phenotype	Number of kernels observed (O)	Number of kernels expected (E)	Genotypes of kernels
Purple and smooth	384	378.56	C_S_
Purple and wrinkle	118	126.19	C_ss
Yellow and smooth	127	126.19	ccS_
Yellow and wrinkle	44	42.06	Ccss
Total	673	673

Test of monohybrid crosses in maize

Record for C₃ kernels

Phenotype	Number of kernels observed (O)	Number of kernels expected (E)	Genotypes of kernels
Purple color	250	254	C_
Yellow color	258	254	Cc
Total	508	508

Test of Dihybrid crosses in maize

Record for C₄ kernels

Phenotype	Number of kernels observed (O)	Number of kernels expected (E)	Genotypes of kernels
Purple and smooth	176	173	C_S_
Purple and wrinkle	168	173	C_ss
Yellow and smooth	170	173	ccS_
Yellow and wrinkle	178	173	Ccss
Total	692	692

Discussion:

The law of segregation state that “allele pairs segregate (separate) during gamete formation, and the paired condition is restored by the random fusion of gametes at fertilization”. from the experiment, I have observed the C maize that followed the mendel firs law. Only 1 traits we observed, only the color of kernels purple or yellow. Mendel also performed crosses in which he followed the segregation of two genes. These experiments formed the basis of his discovery of his second law, the law of independent assortment.

The law of independent assortment state that the law of independent assortment, during gamete formation the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair.” According the C₃ maize, we observed that the kernels have different traits. We observed the color traits and the seed traits. There are four phenotypes, purple and smooth, purple and wrinkle, yellow and smooth and yellow and wrinkle.

As we all know,for each inherited characteristic, an organism has two alleles, one inherited from each parent. Mendel's experiment included on parental variety which had a pair of alleles for green pod color and one which had a pair of alleles for yellow pod color. The F1 hybrids inherited from the parental plants one allele for green pod color and one allele for yellow pod color. A sperm or egg carries only one allele for each inherited characteristic, because allele pairs separate (segregate) from each other during the production of gametes. At fertilization, the sperm and egg unite with both contributing their alleles. This restores the gene to the paired condition. In Mendel's experiment, each gamete of a parental plant carried one allele for pod color, specifying either green or yellow. Cross-pollination to produce the F1 resulted in the combination found in this generation. When the two alleles of a pair are different, one is fully expressed and the other is completely masked. These are called the dominant allele and recessive allele, respectively.

Mendel's hypothesis explains the 3:1 ratio of progeny plant types he observed in the F₂ generation. It predicts that the F1 hybrids (Ff) will produce two classes of gametes when the pairs separate during gamete formation. Half will receive a green-pod (F) allele and the other half the yellow-pod allele (f).During self-pollination, these two classes of alleles unite in a random manner. Eggs containing green-pod alleles have equal chances of being fertilized by sperm carrying preen-pod alleles or sperm carrying yellow-pod alleles. Since the same is true for eggs containing yellow-pod alleles, there are four likely combinations of sperm and eggs. If segregation in the F₁ plants resulted in only two classes of gametes (RY and ry) as were provided from the P generation, then the F₂ generation would have a 3:1 phenotypic ratio (3/4 round yellow seeds and 1/4 wrinkled green seeds). If segregation in the F1 plant resulted in four classes of gametes from each plant because the genes segregated independently (RY, Ry, rY, and ry), then the F+ generation would have a 9:3:3:1 phenotypic ratio (nine round yellow seeds to three round green seeds to three wrinkled yellow seeds to one wrinkled green seed).

Mendel allowed self-pollination of F1 plants (RrYy x RrYy) to produce an F2 generation. When he categorized peas from the F2 generation he found a ratio of 315:108:101:32 which approximates a 9:3:3:1 phenotypic ratio. The phenotypic ratio for each trait singly was still 3:1 (round vs wrinkled and yellow vs green). The experimental results supported the hypothesis that each allele pair segregates independently during gamete formation. Mendel tried all seven of his traits in various combinations in dihybrid crosses and found the same 9:3:3:1 in each case.

From my experiment, based on the result that I get,  the result obeyed the mendel’s law. The first result obey the mendel first law (the law of segregation) while the C₂ obey mendels second law ( the law of independent assortment. The C₃ is test cross( breeding of an organism of unknown genotype with a homozygous recessive) for monohybrid, this ratio is 1:1 which not obey with any mendels law same as the C₄ maize. Mendel only prof the first and second law.

Conclusion:

As a conclusion, we are able to recognize and interpret maize that illustrated Mendel’s both law which are the law of segregation and the law of independent. We also able to state the reasons either the result is relevant or not.

References :

1.      Introduction to genetics

Unit 4 chapter 11

Page 262-275

Kenneth R. Miller

Joseph Levine

Pearson

2.      Biology understanding life

Jon Willey and Son

3.      Biology

Seventh edition

Solomon . berg . Martin

Thomson Brookscole.